1. Field of the Invention
The present invention relates to a technique of resistance welding of a seal opening provided on an end plug of a fuel rod containing fuel pellets and pressurized to a specific gaseous pressure with an inert gas filled through the seal opening.
2. Technical Background
In general, a fuel rod for use in a light water pressurized reactor is made by packing fuel pellets inside a fuel tube, and the ends of the fuel tube are fitted with end plugs and the end plugs are joined to the ends of the fuel tube by means of joining techniques such as the tungsten inert gas (TIG) welding. Further, there is a (gas) seal opening disposed on one of the two end plugs for filling the fuel tube interior with an inert gas under pressure. The gas seal opening is sealed off, by joining methods such as TIG welding, so as to maintain the gaseous pressure of the fuel tube at a specific value.
TIG welding for general welding applications is performed in an open atmospheric pressure, and an inert gas is circulated so as to protect the tungsten electrode and the object to be welded from the chemical effects of the ambient air atmosphere, and to provide cooling.
However, because the welding of the seal opening of a fuel rod must be carried out in a pressurized welding chamber, it is difficult to flow the inert gas between the welding object and the tungsten electrode. It is also difficult to generate the welding arc in the pressurized welding chamber, and it is often necessary to overheat the electrode in the initial stage of the welding operation. Accordingly, when performing TIG welding in a pressurized welding chamber, there is a rapid wear of the tungsten electrode, and depending on the welding conditions, there is a danger of inclusion of the tungsten in the metal of the weld. It is not desirable that tungsten becomes mixed in the weld, because when it happens, the weld quality becomes degraded, for example, by the loss of the corrosion resistance of the weld. Therefore, it was necessary that the welding operation be carried out with utmost care. The tungsten electrode suffered shape changes after each welding operation, due to such factors as melting and vaporization of part of the electrode, thus necessitating adjustments in the arc gap for every welding as well as frequent exchanging of the electrode for grinding of the electrode so as to reshape the tip to the required shape.